Lobster swimmerets: Muscle fiber composition and membrane definition of excitatory synapses in a predominantly fast system

Abstract

AbstractThe abdomen of the lobster Homarus americanus has four pairs of segmental appendages, the swimmerets, whose rhythmic beating functions in swimming, righting responses, creating water currents, and aerating eggs. There are 24 separate muscles in each swimmeret and 21 of these are fast, based on their relatively short (4 μm) sarcomeres and six thin filaments orbiting a single thick filament. Three of the 24 muscles are slow with relatively long (7–12 μm) sarcomeres and 8–11 thin filaments surrounding a thick filament. Of the eight functional groups amongst which the 24 muscles are distributed, six groups have only fast muscles, one group has a mixture of fast and slow muscles, and one group only slow muscles. Thus the number and distribution of fast muscles in the swimmeret makes it a predominantly fast system. Motor innervation of two fast muscles in the power‐stroke group reveals numerous excitatory nerve terminals (recognized by circular synaptic vesicles) with well‐defined synaptic contacts and presynaptic dense bars or active zones. Synaptic contacts appear in the P‐face of the fractured nerve membrane as circular plateaus with sparsely distributed particles and one or two small aggregations of large intramembranous particles (putative calcium channels) representing active zones. In the complementary E‐face, synapses assume spherical outlines and active zones appear as shallow depressions. Attachment sites of synaptic vesicles appear as large pits in P‐face or protuberances in E‐face leaflets along the edge of the active zones. The surface features of excitatory synapses of fast swimmeret muscles are similar to those on other crustacean fast and slow muscles. The packing density of large intramembranous particles in the active zones also resembles that in other crustacean muscles thereby suggesting an optimal packing density for these ion channels. © 1995 Wiley‐Liss, Inc.